The present invention relates generally to a high frequency power amplifier module in a multi-stage configuration having a plurality of cascaded semiconductor amplifiers (variable power amplifier) and a wireless communication apparatus incorporating this high frequency power amplifier module, and particularly, to the technology which is effectively applied to improve the controllability for the output power by a control voltage.
More specifically, the present invention relates to a high frequency power amplifier module of a type which changes the transconductance by a gate bias for changing the output power of the high frequency amplifier module without a significant increase in power consumption, wherein the high frequency power amplifier module comprises a gate bias control circuit for controlling the gate bias, i.e., a drain current, for use in improving the control lability for the output power by a control voltage.
Mobile communication devices, for example, wireless communication apparatus for use in automobile telephones, portable telephones and so on typically have a built-in high frequency power amplifier module, i.e., a high frequency power amplifier circuit in a transmission output stage thereof. The high frequency power amplifier module is configured to automatically control the output (transmission power) by an APC (Automatic Power Control) circuit.
For example, a high frequency power amplifier module having a plurality of cascaded MOSFETs (Metal Oxide Semiconductor Field-Effect-Transistor) as semiconductor amplifier devices has been widely employed up to now because of its convenience in handling, resulting from the fact that the output can be readily controlled by a positive voltage (for n-channel MOSFETs) biased to the gate.
JP-A-7-94975 discloses a three-stage high frequency HIC (Hybrid IC) module, i.e., a high frequency power amplifier module, which has MOSFETs cascaded in the first, middle and last stages.
This high frequency HIC (hybrid IC) module comprises a first bias circuit configured to bias a gate of a predetermined MOSFET out of MOSFETs in a plurality of stages, based on an output control voltage; a second bias circuit for biasing gates of the remaining MOSFETs other than the predetermined MOSFET based on a fixed power supply; and a switching means for switching a path associated with the fixed power supply and a path associated with the second bias circuit in accordance with the output control voltage. This configuration is intended to improve the controllability of the output and the efficiency.
Each of the bias circuits is composed of three resistors and a capacitor.
The above-cited literature, however, does not describe any technique for controlling the gate bias, which changes in response to a control voltage vapc, in accordance with the output power.
Generally, a setting for the gate bias which changes in response to the control voltage Vapc is determined by the value of a resistor forming part of the bias circuit, for example, in such a manner that the efficiency is improved as the output power is larger.
The conventional high frequency HIC module changes the transconductance by applying appropriate voltages to the gates of the MOSFETs in the respective stages. This module is advantageous in that a variable output power can be provided in a small circuit configuration without adversely affecting the output power or efficiency.
However, since the gate bias changes at a constant rate in response the control voltage, the bias in each stage for the control voltage cannot be controlled on account of the fluctuating output power of the high frequency power amplifier module, resulting in poor controllability for the output power and inconvenience in utilization. Stated another way, although this high frequency HIC module produces the bias which merely results in good characteristics as a module, it cannot prevent the output power from abruptly changing in response to the control voltage Vapc.
The above cited literature discloses a diagram showing the correlation of the output control voltage Vapc to the output Po. FIG. 16 is a graph which shows the characteristic of an output power Pout versus a control voltage Vapc, similar to the above. This graph was derived from a test conducted by the present inventors.
As can be seen in the characteristic graph, the output power exhibits abrupt rising for a portion of the control voltage, for example, in a range of 1.1 to 1.5 volts, from which it can be understood that there is a problem in controlling the output for practical use.
This abruptly rising region is present when a gate voltage applied to a MOSFET for amplification which lastly operates (turns on) is close to a threshold voltage Vth. This is because, in this region of the gate voltage, the gain of the MOSFET largely varies, and simultaneously the impedance also largely varies, so that a matching loss of a matching circuit largely varies as well.
Further, as the transconductance g, is improved due to improved performances of recent devices, MOSFETs used in respective stages present larger gain fluctuations with gate bias, resulting in a tendency to more abrupt output power fluctuations and lower output controllability.
It should be noted that the foregoing problem of poor controllability for the output is not limited to the MOSFET but is common to other semiconductor amplifier devices which have a variable gain in response to a control bias applied to a control terminal.
It is an object of the present invention to provide a high frequency power amplifier module which has good output controllability.
It is another object of the present invention to provide a wireless communication apparatus which comprises the high frequency power amplifier module having the good output controllability.
The above and other objects and novel characteristics of the present invention will become apparent from the description in the specification and the accompanying drawings.
Within various embodiments of the invention disclosed in this application, typical aspects will be described below in brief.
A high frequency power amplifier module (power module) using MOSFETs as semiconductor amplifier devices in accordance with one aspect of the present invention comprises a bias circuit for generating a gate voltage in response to a control voltage Vapc generated based on a power control signal of a wireless communication apparatus to reduce fluctuations in output power in response to the control voltage Vpac in a region near a threshold voltage Vth of MOSFETS in respective amplification stages. This provides a power module which facilitates handling in consideration of the controllability for the output power.
Specifically, the power module controls the output power Pout outputted from an output terminal by controlling a gate voltage which is generated in accordance with a division of the control voltage vapc supplied to a bias supply terminal. The gate bias circuit has a circuit for setting the gate voltage supplied to a control terminal, which is responsive to the control voltage Vapc, so as to change largely in a region where the gate voltage Vg is lower than the threshold voltage Vth of the respective MOSFETs; to change slightly near the threshold voltage Vth; and to present desired power amplification characteristics from the vicinity of the threshold voltage Vth to a high Vapc voltage region.
When the gate bias circuit is applied to a multi-stage power module, the control voltage Vapc serves as an optimal gate bias for each MOSFET through the gate bias circuit. The timing at which the MOSFET in each stage turns on in response to the control voltage Vapc is also set in accordance with fluctuations in the gate bias of each stage with respect to the control voltage Vapc.
Next, a bias circuit for a multi-stage power module will be described particularly in terms of the timing at which the MOSFET in each stage turns on, with reference to FIG. 12 which shows changes in the gate voltages Vg of the MOSFETs in the respective stages in response to the control voltage Vapc.
FIG. 12 is a graph which shows how a first-stage gate voltage Vg1, a middle-stage gate voltage Vg2 and a last-stage gate voltage Vg3 change in response to the control voltage Vapc when the bias circuit is applied to a three-stage power module which has three MOSFETs cascaded in sequence. In the power module having the bias circuit exhibiting the gate voltage characteristics as represented by this graph, the first-stage MOSFET (Q1), which has a slight increase in gate voltage Vg1, is designed to turn from off to on lastly as the control voltage Vapc increases. In this event, the control voltage Vapc is set in a region in which the gate voltage vgl of the first-stage MOSFET, which lastly turns on, fluctuates slightly, while the remaining MOSFETs are set to a gate bias which results in small fluctuations in gain in response to the control voltage vapc. In addition, by turning the remaining MOSFETs on to operate stably prior to the first-stage MOSFET which lastly turns on, the output power of the power module gradually increases from an off-state.
In the power module comprising the bias circuit having the foregoing operation characteristics, the gate voltage Vg largely changes in response to the control voltage Vapc in a region lower than the threshold voltage Vth, and the output power of the power module can be controlled from a low Vapc voltage region, so that a variable range for the output power in response to the control voltage Vapc can be made wide from lower power to high power.
In a region near the threshold voltage Vth, the gate voltage Vg is set to change slightly, thus making it possible to reduce fluctuations in the output power in response to the control voltage Vapc.
In a voltage region higher than the vicinity of the threshold voltage Vth, the bias can be set based on desired high frequency characteristics, so that the improved controllability for the output power will never cause a degraded performance of the power module.
With the ability of setting the timing at which the MOSFET in each stage turns on, prior to the first-stage MOSFET which lastly turns on, the remaining MOSFETs are first turned on to gradually increase the output power from an off-state of the module as illustrated in FIG. 12, so that the controllability is improved in a very low output power region.
Further, the remaining MOSFETs are set in an operation range in which the gain slightly fluctuates when the first-stage MOSFET (Q1) turns on, and fluctuations in the output power are mainly adjusted by a gate voltage Vg1 of the first-stage MOSFET which lastly turns on, so that an abrupt rising of the output power can be mitigated.
As described above, according to the present invention, since fluctuations in the gate voltage Vg is reduced in a region near the threshold voltage Vth where the gain and impedance largely vary in response to the gate voltage Vg, it is possible to limit abrupt rising of the output power and therefore improve the controllability. Also, since the configuration of the present invention can set a gate voltage to simultaneously realize a high output and a high efficiency, the characteristics of the power module will not be degraded. Further, since the output is controlled by controlling the gate voltage, such a control expedient will not result in a reduction in the output power and the efficiency of the power module. In this way, the present invention provides for a good controllability for the output power and convenient handling while maintaining high output power and high efficiency of the power module.
Other objects, features and advantages of the present invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.